For high-frequency applications, bipolar transistors, and in particular heterojunction bipolar transistors (HBT), are currently used. It is known to tune a bipolar transistor for operation with a desired voltage/frequency characteristic. For example, the bipolar transistor may be of a high speed (HS) type that is tuned for best operation to handle high frequency signals. Alternatively, the bipolar transistor may be of a medium voltage (MV) type that is tuned for best operation over a voltage range of, for example, 2V to 3V. Lastly, the bipolar transistor may be of a high voltage (HV) type that is tuned for best operation over a voltage range of, for example, 3V to 8V. Typically, the emitter and base modules are the same across the variety of tuned bipolar transistors. The tuning of the bipolar transistor for a desired application is typically accomplished by modifying the collector module of the transistor.
FIG. 1 is a cross sectional view illustrating an example of a conventional bipolar transistor. At the top surface of a substrate 10, an active area is delimited by isolating structures 12 referred to in the art as deep trench isolation (DTI). A heavily-doped region 14 of a first conductivity type (for example, N type) forming the collector of the bipolar transistor extends in depth in the active area of the substrate 10 delimited by the trenches 12. There is a less heavily-doped layer 16, also of the first conductivity type, at the substrate surface over the region 14. The layer 16 may comprise a layer that is epitaxially grown from the substrate 10. Further isolating structures 18, referred to in the art as shallow trench isolation (STI), delimit the less heavily-doped layer 16 and have a depth which is deeper than a depth of the heavily-doped region 14. Regions 20 for accessing the heavily-doped region 14 of the collector (known in the art as a collector sinker) pass through isolating structures 18. In practice, the regions 20 may comprise a heavily-doped first conductivity type region of the substrate 10.
At the top surface of the substrate 10, a stack is formed comprising an insulating layer 22, for example, an oxide, and a heavily-doped polysilicon layer 24 of a second conductivity type (for example, P type). The stack of layers 22 and 24 extends over the less heavily-doped layer 16 and at least partially over the STI structures 18 on either side of the less heavily-doped layer 16. The layer 16 may be selectively doped, forming a selectively implanted collector (SIC) region 17, with first conductivity type dopant using an implantation through an opening formed in the insulating layer 22. The portion of the layer 22 removed for the making the opening over the less heavily-doped layer 16 is replaced with a stack 25 doped with the second conductivity type comprising a silicon-germanium layer (perhaps including carbon SiGe:C) and a silicon layer. Stack 25 forms the base of the bipolar transistor. The stack 25 may be epitaxially grown from an underside of layer 24 and from the top side of layer 16. An opening is also provided in layer 24, opposite to region 16 and on a smaller surface area than the opening in layer 22. Within the opening defined in layer 24, as well as at the top surface of layer 25, a heavily-doped region 26 of the first conductivity type forming the emitter region of the bipolar transistor is provided. Region 26 is separated from layer 24 by spacers 28 made of insulating material.
An emitter (E) contact 29 is provided on heavily-doped region 26 via a silicide layer 30. A base (B) contact 32 is provided on layer 24 via a silicide layer 34. A collector (C) contact 36 is provided on regions 20 via a silicide layer 38.
The structure of the collector module formed by layer 14, layer 16, and SIC region 17 controls tuning of the bipolar transistor operation to have a desired voltage/frequency characteristic. For example, a bipolar transistor of a high speed (HS) type is controlled by the presence of the selectively implanted collector (SIC) region 17, and the use of a thin collector epitaxy for the layer 16. A bipolar transistor of a medium voltage (MV) type is controlled by omitting the SIC region 17. A bipolar transistor of a high voltage (HV) type is controlled by omitting the SIC region 17 and adjusting some other implantations.
It is recognized by those skilled in the art that there is an advantage to providing a bipolar transistor which exhibits a greater transit frequency. One known way to drive increased transit frequency is to increase the collector doping level. However, tight control must be exercised over the base/collector doping profile in order to minimize the impact of collector doping on the base-collector capacitance. One means for achieving this objective is to replace the N type SIC region 17 in layer 16 in instead use an N type in-situ doped collector layer for layer 16. This in-situ doped collector layer, however, would be present in all bipolar transistors fabricated on the same substrate, and such a structure would accordingly preclude the fabrication of MV and HV type transistors. At this step no masking for epitaxy is possible, this later is applied to all silicon opened region. The dopants level needed to obtain a HS transistor cannot be modified over the wafer. So, no different level of dopants needed to have HS/MV/HV transistors can be simultaneously performed.
Those skilled in the art further recognize that the collector dependent breakdown voltages (BV), such as in the common base configuration (BVCBO) or common emitter configuration (BVCEO), are limited by the collector doping. For example, in the HS type transistor the breakdown voltages are limited by the doping level of the SIC implant, and in the MV type transistor the breakdown voltages are limited by upward diffusion from the buried layer 14 into the layer 16.
There is a need in the art for a bipolar transistor structure and a method for making such a bipolar transistor structure that supports increased collector dependent breakdown voltages for the MV type and HV type of transistors while supporting the fabrication of HS, MV and HV transistors.